Micro-investigation of EPICA Dome C bottom ice: evidence of long term in situ processes involving acid–salt interactions, mineral dust, and organic matter
Introduction
Deep ice cores recovered in central Greenland and on Antarctic sites are considered to be relevant archives of past changes in the climate and the atmosphere's composition for time periods ranging from the last climatic cycle to several hundred thousand years back. While a reliable time–depth relationship is essential to the interpretation of proxy records in terms of paleo-environmental history, ice dating becomes quite uncertain in the lowermost sections of ice cores where time series can be disrupted or altered by flowing ice or by the thermal regime due to the bedrock's proximity. However, and despite the lack of chronology, the exhaustive study of impurities gathered in the deepest layers of polar ice, i.e. close to the ice sheet base and generally referred to as bottom or basal ice, can give innovative clues for understanding the very ancient polar environment and sub-glacial properties. Moreover, closed or open high pressure–high temperature systems encountered at the bottom of polar ice caps can provide a useful illustration for the study of very long term physico-chemical and possibly biological processes in relation with initial ice content and grain growth conditions.
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Information available from previous deep drilling projects
The few studies conducted to date on Greenland and Antarctic bottom ice provide interesting insights on pre-glacial environments and in-situ processes. These are summarized below.
Dielectric profiling (DEP) measurements providing ice conductance and capacitance at a range of frequencies that span the main dielectric dispersion of ice were performed in the 6 m of the basal silty ice sequence from the GRIP ice core (Summit, Central Greenland) and the detailed profile of the high frequency limit of the conductivity (σ∞), the most useful parameter that can be deduced from dielectric measurements, was compared with multi-parameter studies involving water stable isotope, debris content, gas composition of CO2 and CH4 and ion concentrations (Tison et al., 1998). These authors concluded that (σ∞) was fully explained by the intracrystalline conductivity of pure ice solely disrupted by ammonium impurities in the ice lattice, which may have been initially present as gaseous NH3. Whilst ammonium peaks in the higher part of the ice core are related to the deposition of biomass burning products, and among them ammonium formate (Legrand and de Angelis, 1995, Legrand and de Angelis, 1996), carboxylates are largely dominated by oxalate in the basal sequence. The strong correlation between oxalate, ammonium and calcium and the net excess of oxalate compared with what could be expected from uric acid degradation alone led the authors to propose that this basal sequence results from the incorporation of a “local end-term” (firn, permafrost ice) involving local biogenic production by plants and animals that was formed in the absence of the present-day ice sheet.
The first comprehensive study of the ionic composition of accretion ice of Lake Vostok, the largest Antarctic sub-glacial lake, combined with additional isotopic and iron measurements suggested that sedimentary sequences with a composition close to evaporite contribute to the lake chemistry (De Angelis et al., 2004). The second step was to investigate accretion ice using high resolution synchrotron X-Ray micro-fluorescence (De Angelis et al., 2005). Liquid brine micro-droplets (3–10 μm) were observed, that coexist with large irregular sulfur-rich aggregates (10–800 μm) containing gases and a mixture of very fine particles. Most of these objects were sequestered inside large ice crystals that grew slowly after ice formation. Their structure and composition provides evidence of hydrothermal activity at the lake bottom and of haline water pulses carrying fine solid debris perhaps biota from a deeper evaporitic reservoir into the lake. The presence of both reduced and oxidized sulfur forms tightly associated in solid inclusions was particularly interesting regarding potential bacterial activity. The coexistence in ice lattice of relatively scarce large inclusions with regularly scattered micro-droplets can be explained by relocation processes: at high temperature (−3 °C for accreted ice, under in situ conditions), the minimization of grain boundary free energy induces abnormal grain growth, leading to grain boundaries of high crystalline quality (Montagnat et al., 2001). At the beginning of the grain growth process, solid particles initially homogeneously distributed in the ice lattice are probably caught up in moving grain boundaries, in order to reduce free surface energy. However, particles aggregate with progressive grain growth and, when large enough, the aggregates remain in the ice lattice. Considering its high salinity, the brine is likely in a liquid state at −3 °C. Interaction between grain boundaries and liquid inclusions is probably similar to the interaction between ice and water, which means that brine bubbles can remain in the ice lattice.
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The EPICA Dome C core:
The EPICA Dome C ice core (EDC) is one of the two ice cores drilled in the framework of the European Project for Ice Coring in Antarctica (EPICA). The drilling site is located on the East Antarctic Plateau at Concordia Station (75°06′04″S; 123°20′52″ E, 3233 m above sea level), about 1200 km inland. The core reached a final depth of 3260 m estimated to be about 15 m or less above the ice–bedrock interface, based on seismic sounding in the drill hole (Schwander, personal communication). Detailed information on bedrock and surface topography were gained by altimeter and airborne radar surveys conducted in 1994–1995 (Rémy and Tabacco, 2000) and from 1995 to 2001 (Forieri et al., 2004). The most notable bedrock features in the 50 km × 50 km enlargement of the Dome C central area include sub-glacial highlands, a set of north–south-trending parallel valleys tens to hundreds of meters deep and a few cirque bowls. The detailed description of the bedrock under the drilling site provided by Forieri et al. (2004), highlights a topographic depression surrounded by hills on the order of 50–100 m high. Rémy and Tabacco, 2000, and Forieri et al., 2004, concluded that the bedrock topography below the Dome C area developed before the East Antarctic ice cap and was not significantly modified by subsequent glacial erosion, mountain ridges being very likely of tectonic origin while the valleys may have been formed by erosion by wet-based mountain glaciers, weathering of granitic rocks, or karstification of limestone.
While high resolution multi-parameter analyses performed along the upper 3140 m of the core have provided a wealth of paleo-environmental data over the last 8 climatic cycles (see for instance EPICA_Community_members, 2004, EPICA Community Members, 2006, Loulergue et al., 2008, Delmonte et al., 2008, Lüthi et al., 2008, Kaufmann et al., 2010, Wolff et al., 2006, Wolff et al., 2010), the deeper part of the core has remained poorly investigated. The ionic content associated with very sharp sulfate spikes observed between 2800 and 3140 m, was determined by Traversi et al. (2009). Compared with volcanic events recorded in the upper part of the core, these spikes seemed anomalous, with unusually low acidity, high Mg2+ concentrations, high Mg2+/Ca2+ ratios, and significant Mg2+–SO42− correlation. The authors suggest that long term rearrangement of impurities via migration in the vein network led to the formation of soluble magnesium sulfate particles in liquid film at grain boundaries. The dust size profile is not available below 2900 m because of the presence of particle aggregates of unknown origin (Lambert et al., 2008). The first data gained between 3200 and 3260 m reported in Jouzel et al. (2007), show a much lower than expected signal variability of water isotope and deuterium excess records, shared by the oxygen 18 of O2 in air and preliminary dust mass, CH4, and CO2 data, that are however in the range of concentrations found in ice formed under full glacial conditions. Air content comparable to shallower values led these authors to dismiss large scale melting and refreezing as a plausible explanation of the profiles they observed and to suspect that this part of the core has been affected by flow disturbances due to stretching of the ice sequence or mixing of layers of different origins. Based on radiometric ages provided by (234U/238U) activity ratios for a set of samples taken along the EDC core, Aciego et al., 2011, observed a marked change below 3200 m, i.e. in the deepest potentially disturbed ice sequence, with ages ranging from 870 ka, the oldest age measured in the upper part of the sequence, to 85 ka in the deepest sample. They concluded to homogenization of the deep ice prior to resetting of the (234U/238U) in the deepest sample and proposed that the low chemical variability results from vertical ice stretching due to sub-glacial melting and enhanced lateral ice flow while the age resetting in basal ice layers was explained by melting, recrystallization and precipitation processes. They considered as likely that this part of the core consists of ice that spans a relatively short age interval corresponding to the cold stage 20.2. The ice of the last 12 m (i.e. from 3248 to 3260 m) contains visible rounded shaped inclusions, brownish to reddish in color, and increasing in size (from less than 1 mm up to a few mm in diameter) and number density (from less than 10 to more than 20 inclusions per ice section 50 cm long) with depth. These inclusions are generally located at grain boundaries or triple junctions (Tison et al., 2013). Following these authors, this part of the core will be referred to as “basal” ice, from 3200 to 3248 m the ice will be referred to as “deep” ice, and we will use the term “bottom” ice to qualify the whole ice sequence below 3200 m.
In this paper, we present and discuss data gained along the bottom part of the EDC core through a combined analytical approach involving ion chromatography (IC), high resolution synchrotron X-Ray micro-fluorescence (micro-XRF) developed using the European Synchrotron Radiation Facilities (ESRF), and scanning (SEM) and transmission (TEM) electron microscopy. The location of the full set of samples along the bottom ice sequence is summarized in the upper part of Fig. 1. In their companion study (submitted), Tison et al. use part of the IC data in a quite different and complementary way focusing on a systematic comparison of the concentration mean values and distribution frequencies of selected markers in bottom ice lamellae and every previous full glacial episode (i.e. depicting similar water stable isotope ranges) to determine whether a clear paleoclimatic signal can be retrieved in the deeper part of the EDC core. They conclude to a fairly good preservation of the signal in terms of global ice properties but that the time scale has been considerably distorted by chemical stretching due to the increasing influence of sub-glacial topography. We focus here on relocation processes and their link with bio-chemical interactions likely to lead to the formation of large aggregates of soil dust particles, inclusions, and secondary minerals. A careful examination of the full data set makes it possible to highlight some particularities of the bottom ice chemical imprint and provides also information on the very ancient Antarctic environment.
Section snippets
Ion chromatography
IC was applied first to a preliminary set of 9 discontinuous samples and then to high resolution sampling of 10 ice lamellae 25–50 cm long. The 9 samples of the preliminary study were initially ca 8 cm long, 7 of them were taken in deep ice, 2 in basal ice. They were rinsed in 3 successive baths of ultrapure water, a decontamination procedure proved to be efficient even when measuring organic traces in Antarctic ice cores extracted within drilling fluid (De Angelis et al., 2012). However and
In situ processes
The interpretation of species ultimately identified in ice archives requires that the relative influences of processes related to firn/ice aging are disentangled from those occurring in the atmosphere. This is a difficult task not only because these processes are complex but also because similar chemical compounds may be produced through chemical atmospheric interactions as well as in situ interactions.
As a general rule, heterogeneous reactions occurring during transport between the water
Past Antarctic environment and the role of Antarctic sources
Very fine micrometer or sub-micrometer sized alumino-silicate particles were ubiquitous in all bottom ice samples, among them clay, feldspar, quartz, and muscovite particles as commonly found in Antarctic ice cores. This is consistent with a long-range input from Southern Hemisphere continents dominated by Patagonian sources (Delmonte et al., 2010). However, several features that may be considered specific to bottom ice were revealed by ion chromatography measurements and SEM and TEM
Conclusion
Four major findings emerge from the multi-technique approach presented here:
Not only time but above all ice temperature plays a key role in two long term processes taking place in the deepest part of the EDC core. The first is the interaction between acidic species in grain boundaries and alkaline aerosol present in ice crystals and the second is the consolidation of the large wind-borne dust aggregates progressively formed through grain boundary migration, for which we propose here a
Acknowledgments
This work is a contribution to EPICA, a joint European Science Foundation (ESF)/European Commission (EC) scientific program, funded by the EC (EPICA/MIS) and by national contributions from Belgium, Denmark, France, Germany, Italy, the Nederland's, Norway, Sweden, Switzerland and the UK. We acknowledge technical support from the C2FN (French National Center for Coring and Drilling), handled by INSU. We thank all the personnel who have contributed to obtain ice core sampling. We thank Paul Duval
References (91)
- et al.
Toward a radiometric ice clock: uranium ages of the Dome C ice core
Quat. Sci. Rev.
(2011) - et al.
Contributions of an ancient evaporitic-type reservoir to subglacial Lake Vostok chemistry
Earth Planetary Sci. Lett.
(2004) - et al.
Geographic provenance of Aeolian dust in East Antarctica during Pleistocene glaciations: preliminary results from Talos Dome and comparison with East Antarctic and new Andean ice core data
Quat. Sci. Rev.
(2010) - et al.
Formation and occurrence of biogenic iron-rich minerals
Earth Sci. Rev.
(2005) - et al.
Ammonium and non-sea salt sulfate in the EPICA ice cores as indicator of biological activity in the Southern Ocean
Quat. Sci. Rev.
(2010) - et al.
Chemistry of sea-salt particles in the summer Antarctic atmosphere
Atmos. Environ.
(2000) - et al.
Role of bacterial siderophores in dissolution of hornblende
Geochim. Cosmochim. Acta
(2000) A molal-based model for strong acid chemistry at low temperatures (>200 to 298°K)
Geochim. Cosmochim. Acta
(2002)- et al.
High crystalline quality of large single crystals of subglacial ice above Lake Vostok (Antarctica) revealed by hard X-ray diffraction
Comptes Rendus de l'Académie des Sciences (Série II a)
(2001) - et al.
Salt inclusions in polar ice core: location and chemical forms of water soluble impurities
Earth Planet. Sci. Lett.
(2005)
Interaction between smectite and bacteria: Implications for bentonite as backfill material in the deposal of nuclear waste
Chem. Geol.
New MIS 19 EPICA Dome C high resolution deuterium data: hints for a problematic preservation of climate variability at sub-millenial scale in the “oldest ice”
Earth Planetary Sci. Lett.
Occurrence of nematodes, tardigrades and rotifers on ice-free areas in East Antarctica
Pedobiologia
Sea spray aerosol in central Antarctica. Present atmospheric behavior and implications for paleoclimatic reconstructions
Atmos. Environ.
Changes in environment over the last 800,000 years from chemical analysis of the epoch Dome C ice core
Quat. Sci. Rev.
Trace element and individual particle analysis of atmospheric aerosols from the Antarctic Peninsula
Tellus B
Epoch Dome C drilling operations: performances, difficulties, results
Ann. Glaciol.
Exopolymeric substances of sulfate-reducing bacteria: interactions with calcium at alkaline pH and implication for formation of carbonate minerals
Geobiology
Cell concentrations of microorganisms in glacial and lake ice of the Vostok ice core, East Antarctica
Microbiology
Competitive exclusion of sulfate reduction by Fe(III)-reducing bacteria: a mechanism for producing discrete zones of high-iron ground water
Ground Water
Long term trends of mono-carboxylic acids in Antarctica: comparison of changes in sources and transport processes at the two EPICA deep drilling sites
Tellus B
Brine micro-droplets and solid inclusions in accreted ice from Lake Vostok (East Antarctica)
Geophys. Res. Lett.
Volcanic eruptions recorded in the Illimani ice core (Bolivia): 1918–98 and Tambora periods
Atmos. Chem. Phys.
Origins and variations of fluoride in Greenland precipitation
J. Geophys. Res.
Sources of continental dust over Antarctica during the last glacial cycle
J. Atmos. Chem.
Geochimical and microbiological fingerprinting of airborne dust that fell in Canberra, Australia, in October 2002
Geochem. Geophys. Geosys.
Aeolian dust in East Antarctica (EPICA-Dome C and Vostok): Provenance during glacial ages over the last 800 kyr
Geophys. Res. Lett.
Ultrastructural and genetic characteristics of endolithic cyanobacterial biofilmscolonizing Antarctic granite rocks, FEMS Microbiol.
Ecol.
Calcium carbonate as ikaite crystals in Antarctic sea ice
Geophys. Res. Lett.
Brief communication: Ikaite (CaCO3.6H2O) discovered in Arctic sea ice
The Cryosphere.
Evolution of the texture along the EPICA Dome C ice core
Eight glacial cycles from an Antarctic ice core
Nature
One to one coupling of glacial climate variability in Greenland and Antarctica
Nature
New Bedrock map of Dome C, Antractica, and morphostructural interpretation of the area
Ann. Glaciol.
Mesoscale atmospheric circulations over the Southwestern Ross Sea Sector, Antarctica
J. Appl. Meteor.
An investigation by analytical transmission electron microscopy of individual insoluble microparticles from Antarctic (Dome C) ice core samples
Tellus
Mineralogy of insoluble particles in the Vostok Antarctic ice core over the last climatic cycle (150 kyr)
Geophys. Res. Lett.
Stable prenucleation calcium carbonate clusters
Science
Crystallization and characterization of magnesium methanesulfonate hydrate Mg(CH3SO3)2.12H2O
Crystal Growth Des.
Atmospheric microbiology in the northern Caribbean during African dust events
Aerobiology
Variations of constituents of individual sea-salt particles at Syowa station, Antarctica
Tellus
Sulfate in air and snow at the South Pole: implications for transport and deposition at sites with low snow accumulation
J. Geophys. Res.
Terrigenous sedimentation processes along the continental margin off NW-Africa: implications from grain size analyses of surface sediments
Sedimentology
A relationship between ion balance and the chemical compounds of salt inclusions found in the Greenland Ice Core Project and Dome Fuji ice cores
J. Geophys. Res.
Sulphate-climate coupling over the past 300,000 years in inland Antarctica
Nature
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